Mask creation with hierarchy management using cover cells

ABSTRACT

A method and apparatus for translating a hierarchical IC layout file into a format that can be used by a mask writer that accepts files having a limited hierarchy. Cover cells of the original IC layout file or a modified file are designated, and the hierarchical file is redefined to include only those designated cover cells. Non-designated cover cells and other geometric data are flattened into the designated cover cells. The hierarchy of the modified file is then redefined to be less than or equal to the hierarchy limit of the mask writing tool.

FIELD OF THE INVENTION

The present invention relates generally to photolithographic processingand in particular to methods and apparatus for creating files thatdescribe devices to be created with one or more photolithographicmasks/reticles.

BACKGROUND OF THE INVENTION

The vast majority of all complex integrated circuits (ICs) are createdby circuit designers using computers. Most circuit designers usecomputer programs to define the functionality required of the circuitand the computer analyzes the functionality requested in order to createthe electronic equivalent of a circuit diagram.

To convert the designer's intent into a physical, integrated circuit,other computer programs take libraries of cells representing groups oftransistors and other low level circuit components that provide thedesired functionality, determine locations for these components, andconstruct the wiring necessary to interconnect them. Such a tool isoften called a “place and route tool.” Custom design tools also exist toprovide other methods to craft complete IC layouts. Once all theelectronic devices have a physical representation, it is expected thatthe physical circuit will deliver the specified electrical performance.The layers of the layout data are fabricated as a set of masks/reticlesthat are used in the photolithographic processing of the actual circuitsthemselves.

Before translating the IC layout data into a format for use by aphotolithographic mask or reticle writing tool, the IC layout data areoften analyzed by one or more other computer programs to ensure that nodesign rules have been broken during the creation of the IC layout dataand/or to correct for errors that can occur during the photolithographicprinting process.

One example of such a program is the Calibre® program produced by MentorGraphics Corporation of Wilsonville, Oreg., the assignee of the presentapplication. The Calibre™ program is a suite of tools that operate onthe IC layout data. These tools include a design rule checking (DRC)program that ensures the compliance with a number of design rulesparticular to the manufacturing process to be used. For example, adesign rule can specify a particular tolerance such as “no transistorscan be located within x microns of other transistors,” etc. In addition,the Calibre® program can perform optical process correction (OPC) tocompensate the layout for distortions that are likely to occur duringthe printing of the photolithographic mask or reticle. Calibre® can alsoperform phase shift mask (PSM) modifications that add phase shifters tothe mask or reticle in order to enhance contrast between features or addsubresolution features on an integrated circuit.

After verifying and/or correcting the layout data, the data aretranslated into a format that can be utilized by a mask or reticlewriting tool. Examples of such formats are MEBES, for raster scanningmachines manufactured by ETEC, an Applied Materials Company, “.MIC”format from Micronics AB in Sweden for their mask writers, and variousvector scan formats for Nuflare, JEOL, and Hitachi machines. Oncewritten, the masks or reticles are then used in a photolithographicprocess to expose selected areas of a silicon wafer in order to producethe integrated circuit components on the wafer.

Many mask writing tools require file formats that are “flat,” whereineach object to be created on a mask is separately defined in the file.Computer files written in a flat format containing the corrected IClayout data can be enormous. For example, one IC layout data file for asingle layer of a field programmable gate array can be approximately 58gigabytes long. The time required to transmit a file of this size to amask or reticle writing tool with standard network protocols can exceed60 hours. When such large files are transmitted over communicationnetworks, the risk that an error will occur during transmission riseswith the length of the file transmitted. In addition, the time requiredto transmit the data file can be longer than the time required for themask or reticle writer to produce a mask or reticle from the file.Therefore, the mask writing tool is inefficiently used when the datafiles are too large.

To speed processing, some mask writers are accepting IC layout fileformats that have a limited number of hierarchy levels permitted.Instead of requiring a separate description of each placement of anobject to be created on a mask, a hierarchical file can includereference to objects or groups of objects that are placed at more thanone location on the mask. This hierarchical description of objects to becreated saves considerable memory and improves processing time. Calibre®uses a hierarchical database to analyze IC layout files. For example,the layout format GDS-II has no limit on the number of hierarchy levelspermitted. Despite the advantages of a hierarchical description, mostmask writing tools do not allow as many levels of hierarchy as an ICverification program. Typical mask writers may only allow a few levelsof hierarchy. Therefore, the IC layout files must be converted to aformat that can be used by a mask writing tool. If the conversion is notdone efficiently, however, many of the advantages of the originalhierarchy can be lost.

Given these problems, there is a need for an improved method oftranslating hierarchical IC layout data into a format having fewerlevels of hierarchy such as for use by a photolithographic mask orreticle writing tool in a manner that reduces file size, improvesprocessing speed and retains at least some of the advantages of theoriginal hierarchical description.

SUMMARY OF THE INVENTION

A system and method for translating a hierarchical IC layout file into aformat that can be used by a mask writer that accepts hierarchical filesof limited depth. The system designates cover cells from the original ormodified hierarchical data and extends the designated cover cells toinclude the geometric data and the cells that were not designated tocreate a modified hierarchical file. The hierarchy of the modified fileis then selectively redefined such that it meets the depth requirementsof the mask writer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIGS. 1A-1B illustrate a simplified IC layout and its correspondinghierarchical graph;

FIGS. 2A-2C illustrate one method of redefining the depth of ahierarchical graph to comply with the maximum file depth requirements ofa mask writing tool;

FIGS. 3A-3C illustrate a method of redefining a hierarchical graph withselected cover cells in accordance with one aspect of the presentinvention;

FIGS. 4A-4C illustrate a method of expanding designated cover cells tocomply with maximum file depth requirements of a mask writing tool inaccordance with another aspect of the present invention;

FIGS. 5A-5D illustrate an alternative method of redefining the hierarchyof a hierarchical graph in accordance with another aspect of the presentinvention; and

FIG. 6 illustrates an exemplary system for implementing an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As indicated above, the present invention is a method and apparatus forredefining the hierarchy of an IC layout data file in order to complywith the file requirements of a mask writer or other tool. The IC layoutdata file can be in any of a number of standard formats such as GDS-II,OASIS, CIF or a previously fractured file such as MEBES or other formatused by a mask writer. FIG. 1A shows a simplified integrated circuitlayout including a plurality of cells A, B, C . . . I that are definedwithin a top cell T. Each cell can include collections of individualgeometries to be formed on a mask/reticle as well as references to othersuch collections. As can be seen, the top cell T includes placements oflarger cells D and H that in turn include placements of repeated cellsB. Each instance of Cell B includes a placement of a repeated cell G.

FIG. 1B illustrates a hierarchical graph that represents therelationship between the top cell T and the cells A-I of the layoutshown in FIG. 1A. Each cell referenced by the top cell T has a maximumdepth represented by the longest path that extends between the top cellT (defined at level zero) and the cell in question. For example, cell Bhas a depth of three if referenced through cells A and H, or a level ofone if referenced directly from the top cell T. Therefore, cell B has amaximum hierarchical depth of three. Similarly, cell G has a maximumhierarchical depth of five since the longest reference path is throughcells H, A, B, C.

In many instances, the hierarchical graph used to describe the IC layoutdata has more levels than can be accepted by the mask writing tool thatwill create the corresponding masks/reticles in order to fabricate theIC layout data on a silicon wafer. Therefore, the hierarchical graphmust be transformed to comply with the requirements of the mask writingtool. Many mask writing tools will only accept flat files, wherein eachobject to be created on the mask must be separately defined in the file.However, some new mask writing formats will accept limited levels ofhierarchy, such as up to 10 levels of hierarchy, etc.

FIGS. 2A-2C illustrate one simplistic method of redefining the hierarchyof the integrated circuit layout data to a format that is acceptable tothe mask writing tool. The original IC layout hierarchical graph shownin FIG. 2A is compressed by first flattening deep placements of cellswithin the hierarchy. The notation A′ designates the cell A that hasbeen modified etc. In the example shown, the original hierarchy has amaximum depth of five, and the graph shown in FIG. 2B is modified suchthat all placement of cells with a depth greater than a maximum depthlimit of two are flattened as shown in FIG. 2B. Furthermore, anyunacceptable cells can be expanded into their parent cells therebyproducing the resulting hierarchical graph as shown in FIG. 2C.Expanding a placement of a cell typically entails replacing thereference to the cell with the contents of the cell—geometry and childplacements—transformed to the correct location. What constitutes anunacceptable cell is usually defined by criteria dictated by the maskwriting tool and can depend on such factors as the geometric areaoccupied by a cell or the memory required to describe a cell. Forexample, if the geometric area of a cell is too big, the beam deflectionmechanism of the mask writing tool may not be able to produce the cellin a single step.

While the hierarchical data graph shown in FIG. 2C meets therequirements of a mask writing tool having a maximum of two levels ofhierarchy permitted, many of the advantages of the original cellhierarchy can be lost.

A better way of redefining the hierarchy of the original IC layout datato comply with a format that is acceptable to the mask writer involvesthe careful designation of cover cells used to represent the data in thehierarchy. The cover cells should be designated to minimize the maskwriter operation time as well as minimize the time required to createthe modified data. Furthermore, depending upon the requirements of themask writer, the mask writer input file size should be minimized, andthe specifications and recommendations for the format for cells in theirplacements should be followed.

In one embodiment of the invention, each of the cells in the IC layoutdata are ranked according to the geometric area occupied by the combinedplacement of each of the cells and the number of times a cell occurs inthe layout.

If the designated cells are too small, the time required to write allthe placements of the cells on the mask increases. On the other hand, ifthe memory required to store a description of a designated cell is toolarge, then the time to transfer its description to the mask writingtool is too long. Finally, the area occupied by the selected cells onthe mask should be maximized to reduce redundant geometry and thecorresponding file size and processing time. It is against thesecompeting criteria that cover cells of the IC layout are designated.

The set of cells that best meets these criteria may be optimallydesignated by considering every set of cells of every possible size.This exact method may be computationally time-consuming. Approximationmethods may therefore be used but run the risk of designating asuboptimal set. In practice, the approximate methods set forth in thedescribed embodiment of the invention, work very well, often as well asthe exact method, and with much less computation.

One such approximate method of designating cells is to evaluate eachcell in accordance with a cost function. In an actual embodiment of theinvention, the cost function is set forth in Equation 1: $\begin{matrix}{{Mi} = {{K \cdot \frac{Ci}{{{area}\quad{left}}\quad}} + \exp^{- {(\frac{{\frac{Ai}{chiparea} - M}}{S})}}}} & (1)\end{matrix}$where “Mi” is the cost function result of a given cell, “Ci” is thecoverage for the cell, i.e., the total area occupied by all instances ofthe cell within the integrated circuit. The term “chip area” refers tothe total area of the integrated circuit. The term “area left” is thechip area less the area occupied by any previously selected cells. Theterm “Ai” is the area of an individual instance of the cell underconsideration. Cell area may be measured as simply the size of therectangular extent of the cell. A more accurate measurement reflects thearea of the cell not overlapped by other cell placements in any of itsplacements (“template-specific area”), and the geometrical complexity ofthe data within that area. The term “M” represents an ideal cell area,which, in one embodiment of the invention, is about {fraction (1/250)}of the area of the integrated circuit and the term “S” characterizes theuseful range of cell area values around M.

Each cell in the IC layout is given a score for the cost function andthe highest scoring cells are designated as the selected cover cells.

The following is a pseudo-code listing of a computer program thatimplements the selection of cover cells according to the cost functionset forth in Equation 1. Inputs: ------- hierarchical database Containsgeometry, overlap areas, and the hierarchy. Algorithm: ---------- //First, allocate storage space for various values for each cell.AllocateArray a[ ]; // Template specific area for each cell.AllocateArray n[ ]; // Number of flat placements of each cell.AllocateArray c[ ]; // “Coverage” for each cell. // Compute initialvalues for a[ ] and c[ ]. for each (cell in database) {  a[i] =EXTENT(cell) − OVERLAP_AREA(cell); // The template-  specific area. n[i] = number of flat placements of cell (from database);  c[i] =n[i] * tsArea; } // Next, cover the chip area with placements ofindividually selected cells. // After each cell is selected, update thearea and placement count records // for all remaining (not yet selected)cells. stop = false; areaLeft = total chip area; coverCells = { }; while(stop == false) {  maxScore = 0;  maxCell = 0;  nTotal = 0;  NL = 4; NU= 300; // Empirically chosen constants. // The cell mark is a generalpurpose database flag.  for each (cell in database) {   if (NL <=n[cell] <= NU) clear cell mark;   else set cell mark;  }  for each(unmarked cell in database) {   m = Score(c[cell], a[cell], areaLeft);  if (m > maxScore) {    maxScore = m;    maxCell = cell;   }  } coverCells = coverCells UNION {maxCell};  nTotal = nTotal + n[cell]; areaLeft = areaLeft − c[cell];  UpdateCoverage(c, a, maxCell,database);  stop = UpdateStop(areaLeft, nTotal, maxCell); } // Thecoverage score function. Score (ci, ai, areaLeft) {  // Empiricallychosen constants.  K = 2;  M = 0.004;  S = 0.01;  return (K *(ci/areaLeft) + exp {−(|ai/chipArea− M|) / S}; } // The stoppingcondition update function. UpdateStop (areaLeft, n, maxCell) {  //Empirically chosen constants.  MAX_COVER 40;  MAX_PLACEMENTS = 400; AREA_LEFT = 0.05  // Stop if the number of selected cells is too large,the number  // of total placements is too large, or enough area has beencovered.  return true if ((|coverCells| > MAX_COVER) OR      (nTotal >MAX_PLACEMENTS) OR      (areaLeft/chipArea < AREA_LEFT));  else returnfalse; } // The coverage update function. UpdateCoverage (c, a,coverCell, database) {  // First, mark cells that contain the chosencover cell.  // Disqualify these cells as future cover cells.  //“Previous” means previous in the topological sort sense.  //Alternatively, one could modify a[i] to account for  // placements ofthe cover cell, but that is less efficient.  mark (coverCell);  for(currentCell = coverCell->previous; currentCell != TOPCELL;     currentCell = coverCell->previous) {   if (any placement ofcurrentCell is marked) mark(currentCell);  }  // Now reduce coverage forcells that have placements in the  // cover cell.  // “Next” means nextin the topological sort sense.  AllocateArray nn[ ]; // Number ofplacements of each cell in the cover  cell.  initialize nn[ ] to zero; for (currentCell = coverCell->next; currentCell != BOTTOM;     currentCell = currentCell->next) {   for each (placement incurrentCell) {     nn[placement's cell] += nn[currentCell];  }  for each(cell in database) {    c[cell] −= nn[cell] * n[coverCell];  } }

Although any cell is suitable for consideration as a designated covercell, performance enhancements can be made by only considering cellsthat are repeated more than some minimum number of times in the layoutand less than a maximum number of times in the layout. In oneembodiment, the minimum number of times a cell must be repeated to beconsidered is 4 and the maximum number of times a cell can be repeatedis several hundred.

Although one embodiment of the invention utilizes the cost functiondescribed above, it will be appreciated that there are other ways todesignate cover cells. For example, cover cells could be designatedbased on size alone, such as any cell having {fraction (1/200)}th areaof the integrated circuit could be used. Alternatively, density of acell could be used. Cells having a greater density of polygons take moretime to transmit, so redundant cells having a relatively large area andhigh density could be selected. Manual selection could also be used.

Cover cells can be defined by creating copies of the original cells,modifying original cells such as by flattening or modifying thesubhierarchy of a cell and/or partitioning it into smaller cells orboth. New cells can also be derived from the layout geometry such as bycombining commonly recurring patterns of polygons or groups of polygonsthat can be recognized and represented by a cell placement orcombination of these methods. Added cells can be thought of as new cellsadded to the hierarchy before cover cell designation.

FIG. 3B illustrates a graph of the original IC layout data with cells A,B, C, and I selected as acceptable cover cells. In addition, the topcell T is included as a catch-all cover cell. Once the cover cells aredesignated, the hierarchical graph is redefined so that only these covercells remain as shown in FIG. 3C. The cover cell C becomes cell C′ byincorporating the cells and geometry located in the referenced cell G.Cover cell B becomes cell B′ by referencing the new cell C′. Cell Abecomes cell A′ by referencing the new cell B′ and the top cell Tbecomes cell T′ by referencing the new cells A′, B′, and C′. FIG. 3Aillustrates the hierarchical graph defined solely in terms of theselected cover cells. Each of the designated cover cells meets therequirements for cover cells determined by the mask writing tool.

The redefined hierarchical graph may have a depth that is greater thanthe maximum depth the mask writer allows. Therefore, the hierarchicalgraph must be redefined so that no cell placement has a depth greaterthan the format limit.

One technique, according to an embodiment of the present invention, forredefining the depth of the hierarchical graph is to determine a maximumplacement depth for each cell that is not greater than the hierarchicaldepth limit “L” that is specified by the mask writing tool. For thepurpose of the present specification, this depth is called max_depth. Asshown in FIG. 4B, assuming a depth limit of L=2 for the mask writer, thecell B′ has a depth of 2 if referenced through cell A′ and a depth of 1if referenced through the top cell T′. Therefore, the max_depth for cellB is 2. Cell C′ has a depth of 3 if referenced through cover cells A′and B′ and a depth of 1 if referenced from the top cell T′. Becausedepth 3 exceeds the depth limit L=2, cell C′ has a max_depth of 1.

Once the maximum placement depth not greater than L is calculated foreach cover cell, a topological sort of the hierarchical graph is made.This is a list of the cover cells ordered such that no cell is seenbefore any of its parents. The list is then analyzed in the reverseorder, i.e., starting with the lowest depth cover cell and workingtoward the top cell. For each cover cell in the list, any placement of asubgraph for that cell is expanded if the placement has a depth greaterthan L-max_depth in the subgraph. For example, cover cell C′ has anempty subgraph. Therefore, the cover cell C′ does not need to bemodified. Cover cell B′ has a max_depth of 2 and L-max_depth=2−2=0.Therefore, any subgraph for cell B′ can have a depth no greater than 0.A placement of cell C′ has a depth 1 in this subgraph. Therefore, thisplacement of cell C′ is expanded into its parent cell B′ to producecover cell B” and parent cover cell T.″

The resulting hierarchical graph shown in FIG. 4C is likely to be moreefficient in terms of the time required to transfer to the mask writingtool and the operation of the mask writing tool itself because the covercells are selected according to the criteria set forth above.

FIGS. 5A-5D illustrate another method of redefining the hierarchy of ahierarchical graph. In the hierarchical refinement to achieve a depthreduction, it is also possible to duplicate cover cells to avoidexpanding all placements of a cell. For instance, in FIGS. 5A-5D, onecan duplicate cell B (making cell B1) and place the copy in one of theplacements, effectively making it a different cell. After expandingplacements of noncover cells, the cell B1 has a subgraph of acceptabledepth while the other placements of cell B must be expanded. Thus onlycell B must be expanded. This may improve the hierarchical impression ofthe final file.

FIG. 6 illustrates one possible system for producing the redefinedhierarchical graph in accordance with the present invention. An IClayout file 100 is received on one or more computer readable media 100or via a wired or wireless data communication link by a stand-alone ordistributed computer system 102. The computer system 102 reads a datastorage medium 101 such as a CD or internal hard drive having storedthereon a sequence of program instructions that cause the computersystem 102 to perform the cover cell designation and redefinition of thehierarchical graph to have a depth no greater than a defined depthlimit. In one embodiment, the computer system 102 copies the IC layoutfile into RAM memory and makes copies of the cell descriptions withinthe RAM memory. The copies of the cell descriptions are modified asdescribed above and stored on a computer readable media withoutoverwriting the original cell descriptions to create the redefinedhierarchical graph file. The computer system then transmits theredefined hierarchical graph file to a mask writer 104. The data may betransmitted over a wired or wireless communication link 106 or may betransferred onto computer readable media such as CD's, DVDs, magnetictape or other formats. The mask writer 104 produces one or more masks orreticles 108 corresponding to the IC layout data provided. The computersystem 102 and the mask writer 104 may be located in the same country orin different countries.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the scope of the invention. For example,the cover cell designation and hierarchy refinement can be performedwith any cell of a hierarchy graph defined as a top cell T.Alternatively, an IC layout file could be divided into geographicalregions and each region analyzed independently.

Therefore, it is intended that the scope of the invention be determinedfrom the following claims and equivalents thereof.

1. A method of preparing a file that describes an integrated circuitlayout for use by a mask writing tool that accepts files having alimited hierarchy, comprising: receiving a layout file that definescells arranged in a hierarchical graph; designating a number of covercells in the hierarchical graph; expanding the layout file such thatonly the designated cover cells remain in the hierarchical graph; andafter designating the cover cells, selectively redefining the hierarchyof the hierarchical graph by expanding and/or copying one or more of theremaining cover cells such that the hierarchical graph has a maximumdepth that is less than or equal to the hierarchical limit of the maskwriting tool.
 2. The method of claim 1, further comprising determining anumber of times a cell appears in a layout wherein the cover cells aredesignated according to the number of times a cell appears in a layout.3. The method of claim 1, further comprising determining a memory arearequired to describe a cell, wherein the cover cells are designatedaccording to the memory required for the cells.
 4. The method of claim1, further comprising determining a geometric area occupied by eachcell, wherein the cover cells are designated according to the geometricarea occupied by the cells.
 5. The method of claim 1, wherein thehierarchy of the hierarchical graph is selectively redefined by:analyzing the remaining cover cells of the hierarchical graph in atopological order; and for each cover cell, determining a maximum depthof a subgraph that can be referenced by the cover cell and expanding thecover cell if necessary such that the subgraph has a depth that is lessthan or equal to the allowed maximum depth.
 6. The method of claim 5,wherein the maximum depth allowed for a subgraph is determined bycomputing the difference between the hierarchical limit of the maskwriter and the maximum depth of any placement of the cover cell thatdoesn't exceed the hierarchical limit.
 7. The method of claim 1, furthercomprising creating new cells in the hierarchical graph from groups ofrepeated polygons.
 8. The method of claim 1, further comprising creatingnew cells in the hierarchical graph by dividing or modifying an existingcell in the layout file.
 9. A computer readable media including asequence of program interactions that cause a computer to perform themethod of claim
 1. 10. A file describing a layout of an integratedcircuit that is created by the method of claim
 1. 11. A method ofselectively redefining the hierarchy of a hierarchical graph to have adepth that is less than or equal to a defined depth limit, comprising:receiving a hierarchical graph that includes a number of cells, whereinat least one of the cells has a depth that is greater than the defineddepth limit; designating cover cells for the hierarchical graph;redefining the hierarchical graph in terms of the designated cover cellsby expanding the nondesignated cells into the designated cover cells;and analyzing each cover cell in the redefined hierarchical graph todetermine if any placement of the cover cell will cause the defineddepth limit to be exceeded, and if so, modifying a subgraph of a cell tomeet the defined depth limit.
 12. The method of claim 11, wherein theanalysis for each cover cell is determined by: determining if themaximum depth of a cover cell plus the depth of the cover cell'ssubgraph exceeds the defined depth limit, and if so, expanding thesubgraph into the cover cell.